Luminous output transducer



April 21, 1959 s. w. LEVINE 2,832,792

LUMINOUS OUTPUT TRANSDUCER Filed Sept. 10, 1954 2 Sheets-Sheet 1 r43Screen I Genera *0 I S e r/ize INVENTOR BY WM ATTORNEY April 21, 1959 s.w. LEVINE LUMINOUS OUTPUT TRANSDUCER 2 Sheets-Sheet 2 Filed Sept. 10,1954 5. W: Levi/1e INVENTOR BY WW W ATTORNEY United States PatentLUMINOUS OUTPUT TRANSDUCER Samuel W. Levine, New York, N.Y., assignor toFairchild Camera and Instrument Corporation, a corporation of DelawareApplication September 10, 1954, Serial No. 455,117

7 Claims. (Cl. 88-61) This invention pertains to transducers, and moreparticularly to a transducer capable of producing a luminous output inthe form of a dot or spot of light having a predetermined shape butwhose size is a function of a varying electrical input applied to thetransducer.

So-called light valves of a large variety of designs are well known inthe optical and electro-optical arts, finding particular application inmodulating light beams in response to input signals of electrical oracoustic origin. In general, the varying luminous output of such lightvalves consists of changes in the intensity of a beam, for example invariable density sound recording or photographic film. However, lightvalves have also been de-.

vised in which the luminous output is constituted by a focussed line orstrip of light, one of whose dimensions is varied in response to theinput signal. Such types are employed for variable-area sound recording,and when used for push-pull recording, one dimension of the luminousline or strip is altered by equal amounts on opposite sides of a centralray. The present invention aims to provide an optical system comparableto a light valve, but whose luminous output takes the pattern of afocussed geometrical shape (specifically, a square) whose dimensions asto both length and width will vary simultaneously in accordance with aninput signal. In other words, the luminous output may consist of asquare dot of light whose size will be controlled by the input signal tothe transducer, but whose shape factor will remain a geometrical squareregardless of the variations in said signal. However, the dot mayequally well take other geometrical shapes, as will appear hereinafter.

The invention has a particular important application in'graphic arts,since it is ideally and peculiarly adapted to the production ofsuccessive dots of varying sizes which, when laid down in a regularpattern upon a photographic or other photosensitive material, willproduce directly a halftone representation of the informationconstituting the signal input to the transducer. Of course, such signalinformation must be of proper form and periodicity so that separatediscrete dots are produced in regular order, rather than a continuousline whose width only varies with the picture information.

It is a further important object of the invention to provide atransducer of the above type in which the luminous flux of the outputbeam is substantially constant, when integrated over the time ofexposure of one dot, to produce the efiect of a beam of substantiallyconstant intensity whether the dot size called for is large or small.This is necessary to minimize the density variations in the finalproduct or picture, which would otherwise result because of the finitespeed with which the light valve opens and closes.

Without intending to limit the invention thereby, a better understandingof the purposes of the new device may be had from consideration of onesuccessful application thereof. In the making of halft-one printingplates by automatic photoelectric engraving apparatus, it is necessaryto engrave the proper dot structure upon a suitable sheet, the dotsbeing represented by indentations or the like in the sheet material.Such a procedure is well disclosed in U. S. Patent No. 2,575,546, issuedNovember 20, 1951, to J. A. Boyajean. In that particular process, theengraving results from the periodic contact of a hot stylus with aplastic blank, the size of the dot or cavity produced at each point onthe plate being determined by the depth to which the hot stylus enterssuch plate. While that procedure, and the apparatus covered by thepatent, has gone into wide and successful use, the necessity forutilizing a hot stylus imposes limitations which are undesirable in manycases. Similarly, in other engraving techniques Where the product is arepresentation by discrete dots, the necessity for physical movement ofthe cutter or stylus, especially at reasonably high repetitionfrequencies, requires the precise control of a substantial amount ofelectromechanical energy.

Considered as a substitute for a mechanically moving tool or stylus, thepresent invention may be used in connection with a photosensitive plateto produce the desired dot structure in various ways. Thus, if the sheetbeing engraved is a suitable plastic which can be locally hardened byluminous energy, the same may have the desired pictorial representationdeveloped thereon by known processes familiar to those skilled in thephotomechanical aspects of the graphic arts field; e. g., by the use oflight-hardenable materials or the like. Likewise, if the sheet beingtreated is photographically sensitive, a direct halftone picture will beproduced, after exposure of the entire surface to the dot patternproduced by the invention, by the ordinary processes of photographicdevelopment.

In general, then, the invention provides a transducer which produces arecurrent dot pattern as a luminous output, rather than as the result ofthe physical action of a tool. Inasmuch as halftone reproductiontechniques require the use of discrete dots whose order of size is quitesmall (individual dots being not normally resolvable at the intendedviewing distance), the ability to produce dots in a size range Whoselower limit may be of the order of 0.001 inch is desired. On one hand,the solution of the problem is complicated by diffraction effects whichresult when it is attempted to define a small light beam by physicalapertures. Also, the design is made difiicult by the fact thatsuccessive dots, even though of dilferent sizes, must have their centersspaced with great precision; in other words, changes in size of the dotproduced must not involve changes in the centered positions ofsuccessive dots, nor changes in the geometrical shape of the dots, nornoticeable changes in the time integral of luminous flux comprising eachdot when divided by the area of said dot. The invention satisfactorilyaccomplishes all of these aims, and does so by devices which arerelatively inexpensive to manufacture and sufiiciently compact so thatthey may be substituted directly, in many cases, for purelyelectromechanical transducers formerly used.

The invention will best be understood by referring now to the followingdetailed specification of certain preferred embodiments thereof, givenby Way of example, and taken in connection with the appended drawings,in which:

Fig. l is a schematic View, principally in side elevation, of onepreferred form of transducer incorporating the invention,

Fig. 2 is a view in plan showing the shape of an opaque aperture maskused in Fig. 1,

Fig. 3 is a diagram illustrating the shape of the luminous output beamof the Fig. 1 device,

Fig. 4 is a view similar to Fig. 2 of a modified aperture mask,

Fig. 5 is a diagram sirnilar'to Fig. 3 illustrating the instantaneousoutput when using the Fig. 4 mask,

Fig. 6 is a view similar to Fig. l but showing a modified arrangement ofthe parts, and

Fig. 7 is a view in perspective of still another modified form oftransducer.

For reasons which have in part been discussed above, the use of aphysical aperture of variable size is prohibited; such components,including iris diaphragm struc tures and the like, would producesubstantial diffraction effects especially when at their lower limits ofsize, and in addition require multiple moving parts whose masses must beaccelerated at high rates if signals of frequencies ordinarilyencountered are to be reproduced. The mass and friction effects bothinvolve high power requirements which shouldbe avoided. The arrangementshown in Fig. 1 of the drawings requires only asimple stationaryaperture mask, and a single moving part which takes the form of atiltable mirror, but nevertheless satisfies the requirements forvariable dot size and precise centering of all dots. As shown in thatfigure, a light source offsuit: able and constant intensity is indicatedby numeral I0, together with a reflector 12 and a condensing lens system 14 toproduce a concentrated beam of light directed towards theaperture mask 16, and of suflicient size to cover completely theaperture therein. This aperture, as better shown at 18 in Fig. 2, is ofisoscles triangular shape, with an apex angle A of 90 if square dots aretobe produced.

An objective lens 20 is positioned to image theaperture 18 at a knifeedge 22 whose sharp edge protrudes partly into the beam representing theaperture, image shape. However, between lens 20 and knife edge 22, a:reflector 24 is positioned, preferably constituted by a. front-surfacereflectormounted for tilting througha limited angle about an axisperpendicular to the plane of the drawing and lying in the reflectingsurface of the element 24; the'tilt axis should pass through the pointof the reflecting surface at which the central ray of the impinging beamst'rikes'the same, so that tilting movements of the reflector 24will'p'erm'it more or less of the apertureis image at knife edge22 to'beoccluded thereby, without: any sidewise shifting of the shadow edgeproduced by the knife edge.

Suitable means are provided for tilting of reflector 24 in response toelectrical control or input signals. As shown, such means may include acoil 26 energized by the sig{ nals and operating to move an armature 28or the like attachedsecurely to the reflector 24.. The reflector may;be'freel'y pivoted and provided with restraining springs to; return itto its proper zero-deflection position, orthe Jtilt may result fromtorsion or bendingof the reflectorsupm port. Such mechanical details maybe widely variedwithw out departing from the spirit, of the invention.Actually, it is quite feasible to mount the reflector 24 upon the.stylus-carryingpart of the electromechanical transducer shown in'the Boyajean patent, although the powerfcapa bilitles of the lattenare 'greaterthan necessary for the purpose of tilting the reflector, 'which maybeanjex -l tremely light structure. i

From the above, it' follows that the beamshape leav ing the knife edge22constitutes a triangle whose base is. defined by the knife edge, andwhose altitude. line bisects. the 90 degree angle opposite to the base.The base (knife edge) being stationary, the position of the center. ofthe base line is also stationary. The desiIedsquareoHtput dot shape isobtained by combining, along their common. base lines, two suchtriangles, asbest indicated in Fig. 3, wh ch d agrams the shape of thefinal outputbeain. In.

the latter figure, the vertical dot and dash line indicates the magedposition of" the knifeedge with reference .to

the beam'fand 'th'e'triangle to the left or that line.repre-..

sen tsthe beam. shape leaving the physical knifeedgq 22.

The"complementary ,triangle to the rightof the center.

line is produced optically in a mannernowto be, described.

Returning. now .to. .lig. 1, the beam leaving knife edge 22 proceeds toa beam-splitter or semi-reflector 30 disposed at an angle (preferably at45) to the beam direction. One half of the beam proceeds by transmissionthrough 30 to a full reflector 32 and is reflected through a focussinglens 34 onto the surface 36, which may be the photosensitive surface tobe engraved or upon which the succession of dots is to be imaged. Thissurface is shown in perspective for clarity, but it will normally beperpendicular to the plane of the drawing. The other half of the beamproceeds from beam splitter 30, by reflection, to a further fullreflector 38 and is focussed by lens 40 upon surface 36 in a positionadjacent to the spot produced by the. first half of the beam. Since thecommon base line of the two triangular spots is fixed with respect tothe optical axis at the knife edge, and hence with respect to the twoaxes arriving at the surface 36, it follows that tilting mirror 24 willproduce the variable size symmetrical square shape desired, and that thecenter of the square will always fall on the same spot with reference tothe fixed parts of the optical system. This results from the fact thatone half of the beam arriving at surface 36 undergoes a singlereflection (at reflector 32 after leaving the mirror 24), while theother half undergoes two such reflections, one at beam splitter 30 andone atreflector 38. Each reflection reverses thesenseor direction ofeffective beam deflection due to tilting of; 24, whence the use of anumber of reflections differing by one, for the two halves, producesopposite movement ofthe two triangularhalves of the outputdot.

The desired succession of such dots is,- of course, produced-by properrelative movement betweenthe surface 36 and 'the" body of thetransducer, in the usual manner exemplified for instance by the Boyajeanpatent. The, productionof-individual discrete dots in the desiredregularsuccession will require someon-ofli'modulation ofthe lightbeam producingthe dots, e.g., bymodulation of the lightsource 10 or by chopping thebeam itself'at a regular'rate. in the electrical signals applied to coil26, if the. zeroinput position of the reflector is made to correspond toa dot of zero size or less, i.e., to the existence of no light outputwhatever or to a finitesignal for zero light.

Since the principal purpose of the invention is ultimately to providescreen plates made up of discrete dots of predeterminedgshape andvariable size, it is desirable toin-- sure that the exposure whichproduces each such dot is fairly brief as compared with the rate ofmotion of'surface 36 relative to the exposure beam; otherwise, blurringof the dot shape will result. Thus, the light source ;10 may be of atype which can be flashed briefly by peaks or. pulses derived from theenergizing ,voltage source 41, in;

Fig. 1. Alternatively, a steady light source may beemploy'ed, and theon-off modulation obtained by a lightchop'per-or sector disc to give abrief exposure which will the on-ofi modulation will be controlled fromsuch genr The screen generator is indicated schematically, in Fig. l at43, controlling the peaked voltage source 41 as erator.

to frequency,but it will be understood that such a screen generatorwould also provide the necessary frequency control if on-oif modulationis accomplished by a lightchopperor by modulating the signal energyapplied to coil 26.

If" the beam arriving at the aperture maskis homo-, geneous as tointensity, the mask shown in Fig. ;2 will allpwthe centralportionpf alarge dot to be lovers,

The on-ofi modulation mayalso be incorporatedirposed as com ared with asmall dot. 1 The reason for this is that in forming a dot, the devicecommences with the aperture in effect closed, and then allows theaperture to open as reflector 24 is tilted, the maximum tiltcorresponding to the size of the dot being produced. Since the reflector24 has a finite speed of movement, a longer time will be required forthe effective aperture to open to a large size than is required for itto open to a smaller size. Thus in forming a large dot, the centralportion will be exposed for a longer time than the total exposure timeinvolved in making a small dot. To prevent this, it is possible toprovide a decrease in the central light flux which occurs progressivelyas the dot size increases, by employing a special shape of mask shown at42 in Fig. 4. The left profile 44 there corresponds to the aperture 18of mask 16, but the right-hand profile, instead of being a straightline, is the edge of a smaller triangle indicated at 46. Obviously, fordot sizes above a certain value, the projecting point of this opaquetriangle will obtrude upon the beam passing through the mask, and morewill obtrude as the dot size increases. However, below said value, thefull intensity of the original beam will be passed. In effeet, theinstantaneous shape of the dot when of moderate size will then be asshown in Fig. 5, in which the light is passed only in a marginal striparound the square beam cross-section at the surface on which it isfocussed.

. While the arrangement of Fig. 1, with or without the modification asto aperture shape of Fig. 3, performs quite well, and permits theproduction of square output dots as small as a thousandth of an inch onthe side, a preferred embodiment shown in Fig. 6 permits simplificationof the optical parts, is generally easier to maintain in properadjustment, and has a much greater depth of focus. Like referencecharacters in this figure denote the same or equivalent parts as thoseshown in Fig. 1. Thus, numeral again denotes a light source, here shownas a ribbon filament lamp, and a condenser 44 concentrates the lightupon an aperture disc or mask 16, opaque except for the aperture whichmay be of the shapes as shown in Fig. 2 or Fig. 4. An objective lens 20images the light from the aperture, after reflection from tiltablemirror 24, in the plane of the knife edge 22. Tiltable mirror 24 isprovided with drive means as in the previous form of the invention.

From knife edge 22, a collimating lens 48 converts the rays to aparallel beam, which beam enters a prismatic beam splitter generallydesignated by numeral 50. This is formed of two right-angle prisms 52and 54, contacting along one common face as at 56, which interface isprovided with a half-silvered or other beam-splitting layer. The layermay be coated on one or both prism faces or otherwise provided in wayswell known to those skilled in optics. The prism 50 is disposed so thatthe beam enters it normally to one face (of prism 52), and since thebeam is col-limated, aberrations that would otherwise be introduced bythe prism are avoided. One half of the entering beam is thus reflectedat layer 56, then totally internally reflected at the outer surface ofprism 52, and passes to the focussing lens 58, which in this case may bea single lens receiving both components of the spot beam. The other halfbeam, transmitted by layer 56, is reflected only once, namely by totalinternal reflection at the outer surface of prism 54, and is alsofocussed by lens 58 upon the receiving surface 36. Inasmuch as prismassembly 50 is in effect a single integral structure, the alignment ofthe various reflectors, including the beam splitting surface, is readilymaintained. Also, the manufacture of this component involves less costthan the separate manufacture and assembly of the three reflectors ofFig. l, and the arrangement gives a much greater effective depth offocus for such a system.

operationally, the embodiment of Fig. 6 is no dilferent from thatearlier described, so that further elaboration as to this form isbelieved to be unnecessary. The screen generator 43 is here shownapplying ori-ofi modulation to the input signals applied to the drivecoil of mirro 24, as already described.

Both of the forms of the invention described above have employed acommon illuminating system, a common mask and knife-edge and outputbeams which suffer respectively a single reflection, and a pair ofreflections, after leaving the knife edge. A form of the invention willnow be described in which separate illuminating systems are employed, adouble knife edge element, and output arrangements in which only asingle reflector is needed. Bearing in mind that the symmetrical dotshape results from combining beams which have suffered a difference innumber of reflections of one it is apparent that the result can beobtained by reflecting one component beam once only, and the other notat all.

Referring now to Fig. 7 of the drawings, a pair of light sources areindicated at 10 and 11, these being identical with the sourcespreviously described. Each source has the condensing system 14, 15, andeach has an aperture mask 16, 16. These masks are oriented so that theirtriangular apertures have the bases parallel to one another. Separatelenses 20 and 21 focus the beams at knife edge device 22', which dilfersfrom that previously described in having its opposite ends bothsharpened to intersect partly each of the two beams from tiltingreflector 24. The beam from aperture 16 is focussed at one edge of knifeedge 22', and the resulting image is projected to the photosensitivesurface 36 by lens 64. The beam from aperture 16 is focussed at theother edge of knife edge 22 after reflection from the front: surfacemirror 60 to reverse the angular sense of rota-- tion of the beampassing from mirror 60 through lens. 62 with respect to the angularrotation of the beam pass-- ing through beam 64. Thus, for example, asthe mirror rotates counter-clockwise, a greater portion of the projected aperture 16 is intercepted by the knife edge and one-half of theresulting square is decreased in size. This same counter-clockwiserotation of mirror 24 decreases the eifective size of the projectedaperture 16' because of the reversal in angular direction produced bythe reflection by mirror 60.

' The optics are arranged so that the base lines of the variable sizetriangles projected on to surface 36 are coincident, so that these twoimages combine to form a single dot or spot of square shape and variablesize just as in the previous forms of the invention.

One advantage of this form of the invention lies in the fact that sinceno splitting of the original beam occurs, a given light source intensityand optical components permits about twice as much light to be directedinto the resulting spot, because light is transmitted throughout thesystem from each of the two sources. Fundamentally, however, the sameoptical principle (differential number of reflections) is employed.While the double knife edge element 22' is shown as an integral element,it is obvious that separate knife edges could be used so long as theyare located in fixed positions corresponding to the knife edges at theopposite ends of the single element 22, or in other fixed positionswhich will produce the same optical effect.

While the invention has been described in connection with certainpreferred examples in accordance with the patent statutes, the detailscan be varied in many respects without departing from the spirit of theinvention. Thus, whereas the aperture masks shown herein comprise opaqueelements with triangular apertures or transparent parts, combined toprovide a truly square output shape, other output shapes of symmetricalconfiguration can be obtained by using different masks. A slightdeviation from true square shape may be desired, for example, whererelative movement between the transducer output axis and the surface 36would tend to elongate the dot in the direction of surface movement, ormore especially where the projection angle onto surface 36 is other than90?; 'I he square dots may also-be intentionally oriented--with'respectto the. dimensional directions of surface 36 so as to looklike diamonds or lozenges rather than' squares. Suchalterations andsubstitutions, and changes astoother details,,are intended to be coveredby the invention as deflnediin the appended claims.

. Whatis claimed is:

1. A- luminous output transducer for the preparation of photographicreproductions consisting of discrete half'- tone dot patternscorresponding to-electricalsignals, com prising means forming two beamsof light defininggeometricallyidenticalimagesof a predetermined shapeeach of saidimages having substantial dimensional extension intwoorthogonal directions ina plane perpendicular to periodically duringmovement of said medium to cause the discrete sequential exposure ofspaced points thereof to the varying size pattern of: light to producethedesired half-tone dot representation of said signals.

2; The invention in accordance with claim 1, in which said means forprojecting comprises means defining separate optical paths for saidbeams, and a reflector in at least one ofsaid paths,- andin which thenumber of reflectors in said' respective paths differs by an oddinteger.

3. The invention in accordance with claim 1, in which the means formingsaidtwo beams of light comprises at least one element having atriangular beam-defining? aperture.

4. The invention in'accordance with claim 1, in which the means formingsaid two beams of light comprises at least one mask having achevron-shaped aperture.

5. The invention in accordance with claim 1, in which saidsignal-responsive means comprises a mirror galvanometer.

6. The-invention in accordance with claim 1, in which saidmeans-forprojecting comprises anoccluding element common to said two beamsoflight, and in which. said means for alteringthe sizes of saidimagescomprisesmeans for moving the direction of the rays formingsaid imageswith respect to said occluding element.

7. A luminous output transducer for" the preparation of photographicreproductions consisting of discrete halftone dot patterns correspondingto continuous electrical signals, comprising: a light source, meansdefining an aperture of chevron shape, said light source being disposedto illuminate said aperture, a ray deflector posi-' tioned to receiverays passing through said aperture from said light source, means forrotating said deflector to 7 vary' the direction of the rays reflectedthereby in accordance with said electrical signals, alight occluding,elementhaving one straight edge positioned with respect to saiddeflector so as to intercept more or less of the beam of rays leavingsaid deflector, on a line connecting the legs of said chevron, opticalmeans for dividing said beam into two'components each having raysimaging the shape of said aperture as modified by said occludingelement,

means for focussing said components upon a moving light-sensitive mediumin contiguous relation at the image of said lineto form a succession ofilluminating spots shaped substantially as hollow rectangles of externalsizes determined by the instantaneous position of said deflector, andmeans for periodically interrupting said beam com'' ponents duringmotion of said medium to image upon the latter a discrete series ofspaced dots of varyingsizes,

and of which the dots corresponding to larger external. size receive attheir central region a reduced total inten-- sity of appliedillumination.

References Cited inthe file of this patent UNITED STATES PATENTS2,311,159 Dimmick Feb. 16, 1943 2,436,148 Maurer Feb. 17, 1948 2,518,743Bach Aug. 15, 1950 OTHER REFERENCES Half-tone Stops (Mertl'e), TheAmerican Photoengraver, vol. 21, No. 10, September 1929, pages 937--951, pages 938, 941, 944, 947 relied on.

